The present invention relates to a method for forming a stainproof film on the surface of a silicone elastomer, particularly a silicone elastomer which is formed by curing at room temperature.
Polyorganosiloxane compositions which cure to form an elastomer at room temperature are available in a variety of forms.
Such compositions generally consist of polydiorganosiloxanes terminated with silanol groups and, as a crosslinking agent, an organosilicon compound having hydrolyzable groups such as acetoxy groups, alkoxy groups, dialkyl ketoxime groups, dialkyl amino groups, dialkyl aminoxy groups, and N-methylamide groups, etc. bonded to the silicon atom. These compositions are used in the building, automobile, electrical, and other industries.
In the building industry, these compositions are used in a large quantity for sealing the gaps on walls of concrete or aluminum plates and for glazing and bonding glass plates. The compositions used for this application are generally known as silicone sealants. Because of their outstanding properties, e.g. weather resistance, durability, heat resistance, low-temperature resistance, little change in physical properties with temperature, resistance to ozone and ultraviolet rays, good workability, etc., the demand for silicone sealants is increasing for the construction of high-rise buildings. In addition, the above-mentioned compositions have recently found use as a coating material. When applied to building roofs and walls, they form a silicone elastomeric layer which is superior in water-resistance and weather resistance.
The silicone elastomer obtained from the curable compositions has outstanding properties as mentioned above, but it has a disadvantage since the resulting elastomer is often a little sticky after curing. This stickiness and the good dielectric properties of the major constituent, polydiorganosiloxane, tends to attract dust. This disadvantage is enhanced when the composition includes additives in order to improve the flexibility of the silicone elastomer while maintaining its workability. Such additive may be an inactive polysiloxane which does not participate in crosslinking and migrates to the surface of the formed elastomer. The migrated polysiloxane may then entrap dust, making the dust water- repellent. Such water-repellent dust is not washed away by rain, but stays on the elastomer, and tends to make a building look dirty.
For the purpose of preventing such staining, the present inventors proposed a method for forming a stain resistant film in Japanese Patent Application No. 173443/1979. According to this method, the above-mentioned silicone elastomer is coated with a film-forming composition which is obtained by dissolving a benzene-soluble polyorganosiloxane resin containing trifunctional and tetra-functional siloxane units, the mole ratio of the organic groups per silicon atom in the molecule being less than 2, in a mixed solvent of a hydrocarbon solvent and volatile silane or siloxane. After coating, the film-forming composition is dried and cured.
This process provides a uniform, stain-resistant film on the surface of a silicone elastomer, but the resulting film lacks flexibility and toughness. When applied on the sealant joints or coating materials of high-rise buildings which are subject to vibration, stress, and expansion or contraction due to temperature changes of the substrates, this coating film tends to crack, thereby deteriorating the appearance of the building.
The present inventors subsequently found that a stain-resistant film having flexibility and toughness can be obtained from a film-forming composition which is prepared by dissolving a condensation product of polysiloxane consisting of tetrafunctional siloxy units and mono-functional siloxy units and having hydroxyl groups and/or alkoxy groups bonded to the silicon atom and silanol-terminated polydiorganosiloxane, in a mixed solvent consisting of a hydrocarbon solvent and volatile silane or siloxane. This film-forming composition is dried and cured after application to the surface of a silicone elastomer. The film was more resistant to staining than the silicone elastomer itself, but was not completely satisfactory.
In order to overcome the above disadvantages, the present inventors carried out a series of experiments and completed the present invention which relates to a method for forming a stain-resistant film.